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<pre><code class="lang-java"><span class="hljs-comment">// &#x672A;&#x5B9A;&#x4E49;&#x7C7B;&#x578B;</span>
var x ;
<span class="hljs-comment">// &#x6570;&#x5B57;&#x7C7B;&#x578B;</span>
var y = <span class="hljs-number">5</span>;
<span class="hljs-comment">// &#x5B57;&#x7B26;&#x4E32;&#x7C7B;&#x578B; &#x53CC;&#x5F15;&#x53F7;&#x6216;&#x8005;&#x5355;&#x5F15;&#x53F7;&#x5747;&#x53EF;</span>
var k = <span class="hljs-string">&apos;john&apos;</span>;
var a = <span class="hljs-number">123e5</span>;
<span class="hljs-comment">// &#x5E03;&#x5C14;&#x7C7B;&#x578B;</span>
var b = <span class="hljs-keyword">true</span>;
<span class="hljs-comment">// &#x6570;&#x7EC4;&#x7C7B;&#x578B;</span>
var cars = [];
cars[<span class="hljs-number">0</span>] = <span class="hljs-string">&quot;Saab&quot;</span>;
cars[<span class="hljs-number">2</span>] = <span class="hljs-string">&quot;Volvo&quot;</span>;
<span class="hljs-comment">// &#x5BF9;&#x8C61;&#x7C7B;&#x578B;</span>
var person = {name:<span class="hljs-string">&quot;veng&quot;</span>,age:<span class="hljs-number">22</span>};
var name = person.name;
var name2 = person[<span class="hljs-string">&quot;name&quot;</span>];

var s = Symbol();
</code></pre>
<h2 id="typeof-&#x8FD0;&#x7B97;&#x7B26;">typeof &#x8FD0;&#x7B97;&#x7B26;</h2>
<p><code>typeof</code>&#x8FD0;&#x7B97;&#x7B26;&#x53EF;&#x4EE5;&#x8FD4;&#x56DE;&#x4E00;&#x4E2A;&#x503C;&#x7684;&#x6570;&#x636E;&#x7C7B;&#x578B;&#x3002;</p>
<pre><code class="lang-json"><span class="hljs-comment">// typeof &#x7684;&#x7528;&#x6CD5;</span>
<span class="hljs-keyword">var</span> number = <span class="hljs-number">1</span>;
<span class="hljs-keyword">var</span> string = <span class="hljs-string">&quot;hello&quot;</span>;
<span class="hljs-keyword">var</span> b = <span class="hljs-literal">true</span>;
<span class="hljs-keyword">var</span> un = <span class="hljs-literal">undefined</span>;
<span class="hljs-keyword">var</span> n = <span class="hljs-literal">null</span>;
<span class="hljs-keyword">var</span> o = {};
<span class="hljs-keyword">var</span> array = [];
<span class="hljs-keyword">var</span> func = <span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params"></span>) </span>{};
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">&apos;number:&apos;</span>, <span class="hljs-keyword">typeof</span> number);
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">&quot;string:&quot;</span>, <span class="hljs-keyword">typeof</span> string);
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">&quot;boolean:&quot;</span>, <span class="hljs-keyword">typeof</span> b);
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">&quot;undefined:&quot;</span>, <span class="hljs-keyword">typeof</span> un);
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">&quot;null:&quot;</span>, <span class="hljs-keyword">typeof</span> n);
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">&quot;object:&quot;</span>, <span class="hljs-keyword">typeof</span> o);
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">&quot;array:&quot;</span>, <span class="hljs-keyword">typeof</span> array);
<span class="hljs-built_in">console</span>.log(<span class="hljs-string">&quot;function:&quot;</span>, <span class="hljs-keyword">typeof</span> func);
==&gt;
number: number
string: string
boolean: boolean
<span class="hljs-literal">undefined</span>: <span class="hljs-literal">undefined</span>
<span class="hljs-literal">null</span>: object
object: object
array: object
<span class="hljs-function"><span class="hljs-keyword">function</span>: <span class="hljs-title">function</span>
</span></code></pre>
<h2 id="null-&#x548C;-undefined">null &#x548C; undefined</h2>
<p><code>null</code>&#x4E0E;<code>undefined</code>&#x90FD;&#x53EF;&#x4EE5;&#x8868;&#x793A;&#x201C;&#x6CA1;&#x6709;&#x201D;&#xFF0C;&#x542B;&#x4E49;&#x975E;&#x5E38;&#x76F8;&#x4F3C;&#x3002;&#x5C06;&#x4E00;&#x4E2A;&#x53D8;&#x91CF;&#x8D4B;&#x503C;&#x4E3A;<code>undefined</code>&#x6216;<code>null</code>&#xFF0C;&#x8001;&#x5B9E;&#x8BF4;&#xFF0C;&#x8BED;&#x6CD5;&#x6548;&#x679C;&#x51E0;&#x4E4E;&#x6CA1;&#x533A;&#x522B;&#x3002;</p>
<pre><code class="lang-json"><span class="hljs-comment">//&#x5728;if&#x8BED;&#x53E5;&#x4E2D;&#xFF0C;&#x5B83;&#x4EEC;&#x90FD;&#x4F1A;&#x88AB;&#x81EA;&#x52A8;&#x8F6C;&#x4E3A;false&#xFF0C;&#x76F8;&#x7B49;&#x8FD0;&#x7B97;&#x7B26;&#xFF08;==&#xFF09;&#x751A;&#x81F3;&#x76F4;&#x63A5;&#x62A5;&#x544A;&#x4E24;&#x8005;&#x76F8;&#x7B49;</span>
<span class="hljs-literal">undefined</span> == <span class="hljs-literal">null</span>
<span class="hljs-comment">// true</span>
</code></pre>
<p>&#x65E2;&#x7136;&#x542B;&#x4E49;&#x4E0E;&#x7528;&#x6CD5;&#x90FD;&#x5DEE;&#x4E0D;&#x591A;&#xFF0C;&#x4E3A;&#x4EC0;&#x4E48;&#x8981;&#x540C;&#x65F6;&#x8BBE;&#x7F6E;&#x4E24;&#x4E2A;&#x8FD9;&#x6837;&#x7684;&#x503C;&#xFF0C;&#x8FD9;&#x4E0D;&#x662F;&#x65E0;&#x7AEF;&#x589E;&#x52A0;&#x590D;&#x6742;&#x5EA6;&#xFF0C;&#x4EE4;&#x521D;&#x5B66;&#x8005;&#x56F0;&#x6270;&#x5417;&#xFF1F;&#x8FD9;&#x4E0E;&#x5386;&#x53F2;&#x539F;&#x56E0;&#x6709;&#x5173;&#x3002;</p>
<p>1995&#x5E74; JavaScript &#x8BDE;&#x751F;&#x65F6;&#xFF0C;&#x6700;&#x521D;&#x50CF; Java &#x4E00;&#x6837;&#xFF0C;&#x53EA;&#x8BBE;&#x7F6E;&#x4E86;<code>null</code>&#x8868;&#x793A;&quot;&#x65E0;&quot;&#x3002;&#x6839;&#x636E; C &#x8BED;&#x8A00;&#x7684;&#x4F20;&#x7EDF;&#xFF0C;<code>null</code>&#x53EF;&#x4EE5;&#x81EA;&#x52A8;&#x8F6C;&#x4E3A;<code>0</code>&#x3002;</p>
<pre><code class="lang-js"><span class="hljs-built_in">Number</span>(<span class="hljs-literal">null</span>) <span class="hljs-comment">// 0</span>
<span class="hljs-number">5</span> + <span class="hljs-literal">null</span> <span class="hljs-comment">// 5</span>
</code></pre>
<p>&#x4F46;&#x662F;&#xFF0C;JavaScript &#x7684;&#x8BBE;&#x8BA1;&#x8005; Brendan Eich&#xFF0C;&#x89C9;&#x5F97;&#x8FD9;&#x6837;&#x505A;&#x8FD8;&#x4E0D;&#x591F;&#x3002;&#x9996;&#x5148;&#xFF0C;&#x7B2C;&#x4E00;&#x7248;&#x7684; JavaScript &#x91CC;&#x9762;&#xFF0C;<code>null</code>&#x5C31;&#x50CF;&#x5728; Java &#x91CC;&#x4E00;&#x6837;&#xFF0C;&#x88AB;&#x5F53;&#x6210;&#x4E00;&#x4E2A;&#x5BF9;&#x8C61;&#xFF0C;Brendan Eich &#x89C9;&#x5F97;&#x8868;&#x793A;&#x201C;&#x65E0;&#x201D;&#x7684;&#x503C;&#x6700;&#x597D;&#x4E0D;&#x662F;&#x5BF9;&#x8C61;&#x3002;&#x5176;&#x6B21;&#xFF0C;&#x90A3;&#x65F6;&#x7684; JavaScript &#x4E0D;&#x5305;&#x62EC;&#x9519;&#x8BEF;&#x5904;&#x7406;&#x673A;&#x5236;&#xFF0C;Brendan Eich &#x89C9;&#x5F97;&#xFF0C;&#x5982;&#x679C;<code>null</code>&#x81EA;&#x52A8;&#x8F6C;&#x4E3A;0&#xFF0C;&#x5F88;&#x4E0D;&#x5BB9;&#x6613;&#x53D1;&#x73B0;&#x9519;&#x8BEF;&#x3002;</p>
<p>&#x56E0;&#x6B64;&#xFF0C;&#x4ED6;&#x53C8;&#x8BBE;&#x8BA1;&#x4E86;&#x4E00;&#x4E2A;<code>undefined</code>&#x3002;&#x533A;&#x522B;&#x662F;&#x8FD9;&#x6837;&#x7684;&#xFF1A;<code>null</code>&#x662F;&#x4E00;&#x4E2A;&#x8868;&#x793A;&#x201C;&#x7A7A;&#x201D;&#x7684;&#x5BF9;&#x8C61;&#xFF0C;&#x8F6C;&#x4E3A;&#x6570;&#x503C;&#x65F6;&#x4E3A;<code>0</code>&#xFF1B;<code>undefined</code>&#x662F;&#x4E00;&#x4E2A;&#x8868;&#x793A;&quot;&#x6B64;&#x5904;&#x65E0;&#x5B9A;&#x4E49;&quot;&#x7684;&#x539F;&#x59CB;&#x503C;&#xFF0C;&#x8F6C;&#x4E3A;&#x6570;&#x503C;&#x65F6;&#x4E3A;<code>NaN</code>&#x3002;</p>
<pre><code class="lang-js"><span class="hljs-built_in">Number</span>(<span class="hljs-literal">undefined</span>) <span class="hljs-comment">// NaN</span>
<span class="hljs-number">5</span> + <span class="hljs-literal">undefined</span> <span class="hljs-comment">// NaN</span>
</code></pre>
<h2 id="&#x5E03;&#x5C14;&#x503C;">&#x5E03;&#x5C14;&#x503C;</h2>
<p>&#x5982;&#x679C; JavaScript &#x9884;&#x671F;&#x67D0;&#x4E2A;&#x4F4D;&#x7F6E;&#x5E94;&#x8BE5;&#x662F;&#x5E03;&#x5C14;&#x503C;&#xFF0C;&#x4F1A;&#x5C06;&#x8BE5;&#x4F4D;&#x7F6E;&#x4E0A;&#x73B0;&#x6709;&#x7684;&#x503C;&#x81EA;&#x52A8;&#x8F6C;&#x4E3A;&#x5E03;&#x5C14;&#x503C;&#x3002;&#x8F6C;&#x6362;&#x89C4;&#x5219;&#x662F;&#x9664;&#x4E86;&#x4E0B;&#x9762;&#x516D;&#x4E2A;&#x503C;&#x88AB;&#x8F6C;&#x4E3A;<code>false</code>&#xFF0C;&#x5176;&#x4ED6;&#x503C;&#x90FD;&#x89C6;&#x4E3A;<code>true</code>&#x3002;</p>
<ul>
<li><code>undefined</code></li>
<li><code>null</code></li>
<li><code>false</code></li>
<li><code>0</code></li>
<li><code>NaN</code></li>
<li><code>&quot;&quot;</code>&#x6216;<code>&apos;&apos;</code>&#xFF08;&#x7A7A;&#x5B57;&#x7B26;&#x4E32;&#xFF09;</li>
</ul>
<pre><code class="lang-js"><span class="hljs-keyword">if</span> (<span class="hljs-string">&quot;&quot;</span>) {
    <span class="hljs-built_in">console</span>.log(<span class="hljs-string">&quot;\&quot;\&quot; is true&quot;</span>);
} <span class="hljs-keyword">else</span> {
    <span class="hljs-built_in">console</span>.log(<span class="hljs-string">&quot;\&quot;\&quot; is false&quot;</span>);
}

<span class="hljs-keyword">if</span> ({}) {
    <span class="hljs-built_in">console</span>.log(<span class="hljs-string">`{} is true`</span>);
}
<span class="hljs-keyword">if</span> ([]) {
    <span class="hljs-built_in">console</span>.log(<span class="hljs-string">`[] is true`</span>);

}
=&gt;
<span class="hljs-string">&quot;&quot;</span> is <span class="hljs-literal">false</span>
{} is <span class="hljs-literal">true</span>
[] is <span class="hljs-literal">true</span>
</code></pre>

                                
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